1. Field of the Invention
The present invention relates to a method and an apparatus for tightening bolts by using a bolt tightening device.
2. Description of the Related Art
A conventional method of installing a bearing cap for keeping a crankshaft of an engine to a mounting body, such as a cylinder body, is disclosed in Japanese Unexamined Patent Publication No. H2-41830, for example. This bearing cap installation method includes first to third tightening steps. In preparation for performing these steps, the bearing cap is first fastened to the mounting body by tightening bolts, and curved inside surfaces of the bearing cap and the mounting body are machined to produce a finished bearing bore having a correct round shape. Next, the bearing cap is once removed from the mounting body by undoing the bolts and bolted again to the mounting body with a pair of semicylindrical shell sections of a half-shell bearing together constituting a cylindrical bearing shell fitted in the bearing bore. It is to be noted that each half of the bearing shell has a crush height. In the first tightening step, a linear function representing a relationship between a bolt tightening torque and a tightening angle (bolt turning angle) needed in a process of breaking, or eliminating, bearing crush by fastening the halves of the bearing shell is obtained. In the second tightening step carried out in succession to the first tightening step, a linear function representing a relationship between a bolt tightening torque and a bolt turning angle needed in a process of tightening the bolts for fastening the halves of the bearing shell together with the bearing cap is obtained. In the third tightening step, the amount of additional bolt tightening is calculated by using the two linear functions obtained in the previously performed first and second tightening steps and the bearing cap is further fastened to the mounting body by applying the total amount of bolt tightening obtained by adding the amount of additional bolt tightening to the amount of initial tightening applied in the aforementioned process of machining the bearing bore. As the aforementioned first to third tightening steps are performed in an uninterrupted sequence, it is possible to ensure a high degree of roundness of the bearing bore while leaving a proper amount of clearance between the crankshaft and the half-shell bearing in this conventional method.
As stated above, the amount of additional bolt tightening is calculated by using a combination of the linear function representing the relationship between the bolt tightening torque and the bolt turning angle obtained in the first step of breaking the bearing crush of the halves of the bearing shell and the linear function representing the relationship between the bolt tightening torque and the bolt turning angle obtained in the second step of for fastening the halves of the bearing shell together with the bearing cap in the bearing cap installation method of the aforementioned prior art Publication. According to this bearing cap installation method, the two functions are obtained as precise linear functions. It is therefore possible to precisely determine coordinates of an inflection point X where a line of a first torque gradient a obtained in the first step and a line of a second torque gradient β obtained in the second step join if the first and second torque gradients α, β expressed in terms of the ratio of a bolt tightening torque T to a tightening angle (bolt turning angle) θ of the bolts are both linear as shown in FIG. 17. It is also possible in this conventional method to precisely determine a theoretical seating point θo which is theoretically expected to be reached at the beginning of tightening the bolts as well as the amount of additional tightening θm of the bolts corresponding to an axial force needed to be applied to the bolts for breaking the bearing crush based on the coordinates of the inflection point X and the second torque gradient β.
Therefore, it is possible to exert the same tightening force as exerted in the aforementioned process of machining the bearing bore by controlling the amount of bolt tightening based on the theoretical seating point θo and the amount of final tightening obtained by adding the aforementioned amount of additional tightening θm to the amount of initial tightening corresponding to the amount of tightening applied in the aforementioned process of machining the bearing bore. This makes it possible to set a proper amount of bearing clearance between the crankshaft and the half-shell bearing and to achieve a high degree of roundness of the bearing bore in which the half-shell bearing has been fitted.
However, the first and second torque gradients α, β measured at an actual assembly site where the bolts are tightened tend to be nonlinear. The torque gradient might gradually vary at about a point of transfer from a region of tightening of the first torque gradient α in an initial bolt tightening stage to a succeeding region of tightening of the second torque gradient β as shown in FIG. 6, for example. In such a case, if the second torque gradient β is determined between regions of tightening torques T3, T4 set at the proximity of an inflection point between the lines of the first torque gradient α and the second torque gradient β and a theoretical seating point θb which is theoretically expected to be reached at the beginning of tightening the bolts is calculated based on a line b having this gradient β, it is inevitable that a calculation error corresponding to changes in the torque gradient should occur between the calculated coordinate values of the theoretical seating point θb and the coordinate values of a true theoretical seating point θo′. Consequently, if bolt tightening operation is controlled based on the calculated value of the theoretical seating point θb, the bolt tightening force might be excessive or insufficient, and this would result in an inability to set a proper amount of bearing clearance.
Also, the half-shell bearing may have such a property that the first torque gradient α varies in a nonlinear fashion in the aforementioned process of breaking the bearing crush as shown in FIG. 7. In such a case, if the first torque gradient α is determined at a point where the bolt tightening torque T becomes equal to a specific value and coordinates of a joining point Xc between the first torque gradient α and the second torque gradient β are calculated based on a line c having the first torque gradient α and a line b having the second torque gradient β, it is inevitable that a calculation error corresponding to the aforementioned nonlinear property should occur between the calculated coordinate values of the joining point Xc and the coordinate values of a true joining point X′. Thus, there is such a problem that if the amount of additional tightening θc corresponding to the amount of bolt tightening required for breaking the bearing crush is calculated based on the coordinates of the joining point Xc, the calculated amount of additional tightening θc becomes excessive and an unnecessarily large tightening force would be exerted on the bolts.